Brake apply systems are conventionally known wherein a brake pedal is connected through a push-rod to a power head operating on vacuum assist or hydraulic power. A manual application of force on the brake pedal is transferred through the push-rod to a power head which boosts/increases the amount of force transferred through the brake apply system under the operation of vacuum assist or hydraulic power. The power head applies an intensified force to a master cylinder assembly which draws fluid from an associating reservoir and applies fluid pressure to the braking system. Such a system is travel dependent meaning that braking pressure is a function of travel of the master cylinder and power head.
It is known that this type of conventional braking system can be adapted to operate in traditional base brake modes, anti-lock modes and traction control modes. With most conventionally known ABS systems, master cylinder pressure is used during a brake apply. The master cylinder pressure is transmitted through a modulator that limits the amount of pressure applied to the wheel brake during ABS operation. A motor driven pump is used to pump fluid from the wheel brake(s) to the master cylinder during an ABS release. During traction control operation, there may be no application of the brake pedal and therefore, the booster and master cylinder do not generate a force. Fluid pressure requirements are generated by the motor driven pump. The system's operation during base brake mode is controlled by the booster-assisted master cylinder.
Operation during ABS and traction control modes is generally effected by an ABS/TCS modulator. This modulator is electronically controlled in a programmed manner to limit or provide braking pressure when an event occurs according to preconceived parameters indicative of a preferred ABS or TCS intervention. The base brake mode of operation effects other conventional braking events when ABS and TCS operation is not required. The conventional master cylinder and vacuum booster must therefore, be designed to achieve all potentially necessary base brake operational requirements. This means that in-effect, two parallel, substantially independently operating brake apply mechanisms are provided for the single vehicle braking system.
So-called brake-by-wire systems are also known wherein brake pressure application is not directly generated by force on the brake pedal. These systems apply braking pressure by means of a pump or other pressure generation device in response to a sensed brake pedal application. In such systems it is difficult to make the pedal travel and feel characteristics transparent to the driver since a conventional booster is not employed or if employed, is isolated from the braking system. This can result in a non-preferred pedal feel during brake applications.